THE CALCULATION OF PHOTOIONIZATION CROSS-SECTIONS OF SIMPLE POLYATOMIC-MOLECULES BY L2 METHODS

This article is concerned with the calculation of the photoionisation cross section of molecules due either to the one-photon or, in view of the increasing interest in the nonlinear optical processes, to the two-photon absorption. The emphasis is put upon the approaches which may exploit the extensive wealth of know-how for the calculation of the electronic bound states of molecules to gain useful and detailed information about the electronic continuum. Thus the main attention of this report is devoted to the methods which permit the extraction of properties pertaining to the electronic continuum from L2 basis-set calculations. The first part outlines the features characterizing the electronic continua required to calculate the differential ionisation cross section, and a method, amenable to systematic improvements, based upon the reaction K-matrix to obtain all the required transition matrix elements either from an L2 energy-variational or from an L2 basis-set RPA calculation. Then, following an overview of the problems connected with the implications of the Born-Oppenheimer approximation and with the choice of the gauge, a brief review is presented of the Stieltjes moment theory which permits the evaluation of the oscillator-strength density in the continuum from a discretized representation. The remaining part reports some specific examples of calculations for the one-photon and the two-photon processes.